I think it was ultimately two factors:
a) the Dubs are trying to load up to win another championship as their goal this season. They may not get there, but it's a goal the organization has from the front office, to the coaching staff, to the players. Nothing in the language - whether by words or actions - coming from Brad and Joe suggest they're trying to win a championship this season. The message they're sending with the roster moves is that they want to get below the second apron and then build a new, younger, cheaper team around the Jays next season or the season after. It's entirely possible that both GS and Boston will end up in the same position in the playoffs, but the organizational goals are different and presumably the has less constraints on them.
And b) the Benjamins. Boston is trying to get below the second apron, the Dubs are prepared to spend all the way up to the 2nd apron and maybe even exceed it during the season to maximize Curry's waning light. We were never going to be able to match the $$$ they were giving him.